Golf ball

ABSTRACT

In a golf ball having a core and a cover of at least one layer encasing the core, the core is a product molded under heat from a rubber composition which includes (A) a base rubber, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) an organic peroxide. Letting the content of component (B) in the rubber composition per 100 parts by weight of component (A) be X parts by weight and the concentration of volatile substances from component (B) within the core be A wt %, the value A/X is 0.0123 or less. This golf ball has a low energy loss, enabling a high initial velocity to be obtained.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2016-124559 filed in Japan on Jun. 23, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf ball having a core and a cover of at least one layer encasing the core.

BACKGROUND ART

The initial velocity of a golf ball is what drives increases in the distance traveled by the ball, and so it is desirable to raise the initial velocity to the very limit of what is permitted under the Rules of Golf. Golf balls are made of polymeric substances, with the majority of golf balls today being solid golf balls containing a core that is obtained by molding under applied heat a rubber composition prepared by adding a crosslinking agent or organic peroxide, metal oxides and the like to a base rubber such as polybutadiene rubber. The polymeric substances making up the golf ball contain volatile substances such as the following low-molecular-weight compounds: water, various additives and their decomposition products, the decomposition products of catalysts, and residual solvents. JP-B 556-26422 teaches art which, by removing at least fixed amounts of these volatile substances, imparts a golf ball with a higher initial velocity than pre-existing golf balls and is thus able to increase the distance traveled by the ball.

That is, the foregoing art subjects all or part of a golf ball to a given heat treatment so as to remove volatile substances and, by setting the removal ratio thereof to at least 1.0%, increases the coefficient of restitution and the initial velocity. In such art, the percent removal of volatile substances is calculated from the total weight of the volatile substances that volatilize off when the prescribed treatment (heat treatment) has been carried out and the weight of the substances that are subjected to such heat treatment.

However, the foregoing art does not focus on the volatile components which remain within the golf ball. What is actually important is not how much volatile substances have volatilized, but rather, when energy is imparted to the golf ball from the head of a golf club, the degree to which that energy can be converted without loss into initial velocity. Hence, there has long existed a desire for art that reduces the loss of energy.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a golf ball which minimizes energy loss and can have an increased initial velocity.

As a result of extensive investigations, we have discovered that by molding a rubber composition under applied heat to form a core and immediately thereafter heating the core under fixed conditions, the core initial velocity becomes higher when, of the volatile substances, the concentration of unsaturated carboxylic acids and/or metal salts thereof remaining within the core is at or below a given level.

That is, in the golf ball of the invention, which has a core and a cover of at least one layer encasing the core, the core is a product molded under heat from a rubber composition which includes (A) a base rubber, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) an organic peroxide. By heating the resulting core under fixed conditions immediately after the rubber composition has been molded under heat, the amount of volatile substances remaining in the core is suitably reduced, thereby increasing the initial velocity and making it possible to provide a golf ball having an excellent distance performance.

Accordingly, in a first aspect, the invention provides a golf ball having a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition which includes (A) a base rubber, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) an organic peroxide. Letting the content of component (B) in the rubber composition per 100 parts by weight of component (A) be X parts by weight and the concentration of volatile substances from component (B) within the core be A wt %, the value A/X is 0.0123 or less.

In a preferred embodiment of the first aspect of the invention, dicumyl peroxide is used as component (C) in the rubber composition and, letting the content of component (C) in the rubber composition per 100 parts by weight of component (A) be Y parts by weight and the concentrations of acetophenone and α-cumyl alcohol, which are volatile substances from component (C), within the core be respectively B1 wt % and B2 wt %, the value B1/Y is 0.072 or less and the value B2/Y is 0.270 or less.

Preferably, the concentrations of acetophenone and α-cumyl alcohol, which are volatile substances from component (C), are respectively 0.043 wt % or less and 0.15 wt % or less.

In another preferred embodiment of the first aspect of the invention, 1,1-di(butylperoxy)cyclohexane is used as component (C) and, letting the content of component (C) per 100 parts by weight of component (A) be Y parts by weight and the concentration of tert-butanol, which is a volatile substance from component (C), within the core be D wt %, the value D/Y is 0.033 or less.

Preferably, the concentration of tert-butanol, which is a volatile substance from component (D), is 0.033 wt % or less.

In a second aspect, the invention provides a golf ball having a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition which includes (A) a base rubber, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) an organic peroxide. The concentration of acrylic acid, which is a volatile substance from component (B), within the core is 0.234 wt % or less.

In a third aspect, the invention provides a golf ball having a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition which includes (A) a base rubber, (B) an unsaturated carboxylic acid and/or a metal salt thereof, and (C) an organic peroxide. In thermogravimetric analysis of the core, letting (a) represent the percent weight loss at a center of the core and (b) represent the percent weight loss at a surface of the core, the value (a) is 1.31 wt % or less, the value (b) is 1.37 wt % or less, and the value (a)/(b) is 0.96 or less.

Advantageous Effects of the Invention

The golf ball of the invention undergoes little energy loss, enabling the ball to achieve a high initial velocity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description.

The golf ball of the invention has a core and a cover of at least one layer that encases the core. The core in this invention is made of a rubber composition which includes at least (A) a base rubber, (B) an α,β-unsaturated carboxylic acid of 3 to 8 carbon atoms and/or a metal salt thereof, and (C) an organic peroxide.

The base rubber (A) is preferably a polybutadiene. It is advantageous to use as this polybutadiene one having a cis-1,4 bond content on the polymer chain of preferably at least 80 wt %, more preferably at least 90 wt %, and even more preferably at least 95 wt %. When the content of cis-1,4 bonds among the bonds on the polybutadiene molecule is too low, the resilience may decrease. The content of 1,2-vinyl bonds included in the polybutadiene is preferably not more than 2 wt %, more preferably not more than 1.7 wt %, and even more preferably not more than 1.5 wt %, of the polymer chain. When the content of 1,2-vinyl bonds is too high, the resilience may decrease.

From the standpoint of obtaining a molded and vulcanized rubber composition that has a high resilience, this polybutadiene is preferably one synthesized with a rare-earth catalyst or a group VIII metal compound catalyst. A polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Rubber components other than the above polybutadiene may be included in the rubber composition within a range that does not detract from the advantageous effects of the invention. Illustrative examples of rubber components other than the above polybutadiene include other polybutadienes and also other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Illustrative examples of (B) the unsaturated carboxylic acid and/or metal salt thereof include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred. Metal salts of unsaturated carboxylic acids are not particularly limited, and are exemplified by those obtained by neutralizing the foregoing unsaturated carboxylic acids with desired metal ions. Illustrative examples include the zinc salts and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

These unsaturated carboxylic acids and/or metal salts thereof serving as component (B) are included in an amount, per 100 parts by weight of the base rubber, which is preferably at least 5 parts by weight, more preferably at least 10 parts by weight, and even more preferably at least 15 parts by weight. The upper limit in the amount included is preferably not more than 60 parts by weight, more preferably not more than 50 parts by weight, and even more preferably not more than 45 parts by weight. When too much is included, the feel of the ball may become too hard and unpleasant. When too little is included, the rebound may decrease.

Some examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di(t-butylperoxy)cyclohexane, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane and 1,3-bis(t-butylperoxyisopropyl)benzene. Illustrative examples of commercially available organic peroxides include Percumyl D, Perhexa 3M, Perhexa C, Niper BW and Peroyl L (all from NOF Corporation), and Luperco 231XL (Atochem Co.).

The amount of organic peroxide included per 100 parts by weight of the base rubber is preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, even more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight. The upper limit is preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. When too much or too little is included, it may not be possible to obtain a ball having a good feel, durability and rebound.

An inert filler such as zinc oxide, barium sulfate or calcium carbonate may be used in the rubber composition. Such fillers may be used singly or two or more may be used in combination.

In addition, an antioxidant may be optionally included in this invention. Examples of commercial antioxidants include Nocrac NS-6, Nocrac NS-30 and Nocrac 200 (all from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (Mitsubishi Chemical Corporation). These may be used singly or two or more may be used in combination.

When forming the core from the above rubber composition, the various above ingredients may be intensively mixed by a known mixing method, such as one that involves the use of a Banbury mixer, roll mill or other mixing apparatus, and then molded using a core mold.

The core is preferably obtained by additionally subjecting the rubber composition to from 10 to 25 minutes of vulcanization treatment at between 145 and 180° C. This vulcanization temperature is generally at least 145° C., preferably at least 150° C., and more preferably at least 155° C.; the upper limit is generally not more than 180° C., preferably not more than 170° C., and more preferably not more than 160° C. When the vulcanization temperature is too high, the durability decreases; when the vulcanization temperature is too low, the rebound and hardness decrease. The core vulcanization time is generally at least 10 minutes, and preferably at least 13 minutes; the upper limit is generally not more than 25 minutes, and preferably not more than 20 minutes. When the vulcanization time is too long, the productivity decreases; when the vulcanization time is too short, the hardness becomes unstable.

In this invention, by carrying out heat treatment of the vulcanized core under given conditions, the level of volatile substances included in the core (molded and vulcanized product) is ultimately reduced. The core immediately after vulcanization is cooled, and the core surface is subsequently abraded. In this invention, following abrasion of the core, it is desirable to carry out the heat treatment described below.

The conditions when heating the core are a temperature of between 130 and 270° C., and preferably between 150 and 250° C. When this heating temperature is too low, a sufficient ball initial velocity-increasing effect may not be obtained. On the other hand, when the heating temperature is too high, the durability of the ball may worsen.

Heat treatment is carried out for a heating time of from 1 to 120 hours, and preferably from 2 to 48 hours. When the heating time is too short, a sufficient ball initial velocity-increasing effect may not be obtained. On the other hand, when the heating time is too long, the durability of the ball may worsen.

The pressure during heat treatment is preferably atmospheric pressure or below, and more preferably not more than 10,000 Pa. Heat treatment is preferably carried out in an atmosphere of air, hydrogen, oxygen, nitrogen, a noble gas, carbon dioxide, or a mixed gas thereof.

The core has a deflection under a given loading, i.e., when compressed under a final load of 1,275 (130 kgf) from an initial load of 98 N (10 kgf), which is preferably at least 2.0 mm, and more preferably at least 2.5 mm, and is preferably not more than 6.0 mm, and more preferably not more than 5.0 mm.

To effectively elicit the properties of the core, the core diameter is preferably at least 30 mm and especially at least 34 mm, and is preferably not more than 42 mm, and especially not more than 40 mm.

Volatile substances from (B) the unsaturated carboxylic acid and/or a metal salt thereof are present within the core. Specifically, when zinc acrylate has been used as the metal salt of an unsaturated carboxylic acid, acrylic acid is detected as a volatile substance. Likewise, when zinc methacrylate has been used as the metal salt of an unsaturated carboxylic acid, methacrylic acid is detected as a volatile substance. Letting the concentration of this volatile substance be A wt % and letting the content of component (B) per 100 parts by weight of the base rubber (A) in the rubber composition be X parts by weight, the value A/X is 0.0123 or less. When the value A/X is 0.0123 or less, the high initial velocity that is desired can be obtained.

By determining A/X in this way, one can know the ratio of volatile substances from component (B) with respect to the component (B) originally included in the rubber composition. One would expect that when a greater amount of a chemical is included in the composition, a larger amount of volatile substances from that chemical will be present within the core, and so it is difficult to discuss the initial velocity-increasing effect of heat treatment merely in terms of the amount of volatile substances. That is, the degree of increase in the initial velocity is related in some way to the difference between the weight of volatile substances that were present within the core prior to heat treatment and the weight of volatile substances that remain within the core following heat treatment. Hence, the above ratio can be used as an indicator for knowing the degree to which the weight of volatile substances has decreased as a result of carrying out heat treatment, thereby making it possible to discuss the desirable effects even in cores having different compositions.

The above value A/X is preferably 0.0120 or less, and more preferably 0.011 or less.

When the volatile substance from component (B) is acrylic acid, the acrylic acid concentration is preferably 0.234 wt % or less, and more preferably 0.22 wt % or less.

When dicumyl peroxide is used as (C) the organic peroxide component in the above rubber composition, examples of volatile substances from this component (C) include acetophenone and α-cumyl alcohol, which are decomposition products of dicumyl peroxide. Letting the content of component (C) within the rubber composition per 100 parts by weight of component (A) be Y parts by weight and the concentrations of acetophenone and α-cumyl alcohol, which are volatile substances from this component (C) within the core, be respectively B1 wt % and B2 wt %, from the standpoint of increasing the initial velocity, which is the object of this invention, the value B1/Y is preferably 0.072 or less and the value B2/Y is preferably 0.270 or less. Here, the concentration of acetophenone is preferably 0.043 wt % or less and the concentration of α-cumyl alcohol is preferably 0.15 wt % or less.

When 1,1-di(t-butylperoxy)cyclohexane is used as component (C) in the rubber composition, volatile substances from component (C) are exemplified by tert-butanol, which is a decomposition product of 1,1-di(t-butylperoxy)cyclohexane. Letting the content of component (C) in the rubber composition per 100 parts by weight of component (A) be Y parts by weight and the concentration of tert-butanol, which is a volatile substance from component (C) within the core, be D wt %, from the standpoint of increasing the initial velocity, which is the object of the invention, the value D/Y is preferably 0.033 or less. Here, the concentration of tert-butanol is preferably 0.033 wt % or less.

The decrease in volatile substances and the evaporation of moisture owing to the above heat treatment gives rise to a weight loss between the core immediately after vulcanization and the core following such heat treatment. With regard to moisture in particular, it has been confirmed that atmospheric moisture is reabsorbed near the core surface on account of cooling and storage following this heat treatment. However, compared with the core surface, moisture absorption does not readily occur near the core center, and so the vicinity of the core center remains in substantially the same state as that following the above heat treatment. It is therefore possible to detect whether heat treatment has been carried out and the degree of such treatment by looking at the difference between the core center and the core surface in percent weight loss in thermogravimetry.

Specifically, in thermogravimetric analysis of the core, letting (a) represent the percent weight loss at the core center and (b) represent the percent weight loss at the core surface, the value (a) is preferably 1.31 wt % or less and the value (b) is preferably 1.37 wt % or less. Also, the value (a)/(b) obtained by dividing (a) by (b) is preferably 0.96 or less. When (a)/(b) is 0.96 or less, the core can be regarded as fully heat treated, enabling a golf ball core having a high initial velocity to be obtained.

When manufacturing a golf ball using the above core, the ball can be obtained by encasing the periphery of the core with a cover of one, two or more layers. The cover material is exemplified by, but not limited to, ionomer resins and polyurethane resins.

Methods for encasing the core with such a cover are exemplified by the method of pre-molding various cover compositions into hemispherical half-shells, using two such half-shells to envelope the core, and molding for 1 to 15 minutes under applied pressure at between 130 and 230° C.; and the method of enveloping the core by injection-molding such a cover-forming composition directly over the core.

Numerous dimples may be formed on the surface of the inventive golf ball. Also, where necessary, the ball surface may be marked, painted, and surface treated. For competitive play, this solid golf ball can be made to conform to the Rules of Golf. Specifically, the ball may be formed to a diameter of not less than 42.67 mm and a weight of not more than 45.93 g.

The golf ball deflection, i.e., the deflection of the golf ball when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is preferably at least 1.5 mm, and more preferably at least 2.0 mm, but is preferably not more than 5.0 mm, and more preferably not more than 4.0 mm.

The initial velocity of the golf ball is preferably at least 77.0 m/s, more preferably at least 77.1 m/s, and even more preferably at least 77.2 m/s. The upper limit is preferably not more than 77.6 m/s.

EXAMPLES

Working Examples and Comparative Examples are provided below to illustrate the invention, and are not intended to limit the scope thereof.

Working Examples 1 to 19, Comparative Examples 1 to 7 Formation of Core

Cores were produced by preparing the two rubber compositions shown in Table 1, and subsequently molding and vulcanizing the compositions under vulcanization conditions of 157° C. and 15 minutes. In all the Working Examples and Comparative Examples, the cores had a common diameter of 37.70 mm.

TABLE 1 Rubber composition (pbw) I II Polybutadiene rubber 100 100 Zinc oxide 4 4 Barium sulfate 20.92 18.56 Antioxidant 0.1 0.1 Zinc salt of 1 0.4 pentachlorothiophenol Zinc acrylate 18.7 22.6 Zinc stearate 3.3 4.0 Organic peroxide A 0.6 Organic peroxide B 0.24 1.0

Details on the rubber compositions in Table 1 are given below.

-   Polybutadiene rubber: Available under the trade name “BR01” from JSR     Corporation -   Zinc oxide: Available as “Zinc Oxide Grade 3” from Sakai Chemical     Co., Ltd. -   Barium sulfate: Available under the trade name “Barico #100” from     Hakusui Tech -   Antioxidant: Available the trade name “Nocrac NS-6” from Ouchi     Shinko Chemical Industry Co., Ltd. -   Zinc salt of pentachlorothiophenol: Available from Wako Pure     Chemical Industries, Ltd. -   Zinc acrylate: Available from Wako Pure Chemical Industries, Ltd. -   Zinc stearate: Available from Wako Pure Chemical Industries, Ltd. -   Organic Peroxide A: Dicumyl peroxide, available under the trade name     “Percumyl D” from NOF Corporation -   Organic Peroxide B: 1,1-Di(t-butylperoxy)cyclohexane, available     under the trade name “Perhexa C” from NOF Corporation

These cores were heat-treated under the temperature, time, atmosphere and pressure conditions shown in Tables 2 and 3 below. The volatile components remaining in the core following heat treatment and the weight losses at the core center and core surface following heat treatment are also presented in these tables. The heat treatment entailed using a square fixed-temperature vacuum drying oven (DP300, from Yamato Scientific Co., Ltd.) and a small oil-sealed rotary vacuum pump (GLD-136C, from Ulvac Kiko, Inc.), arranging 30 cores side by side within the oven, and carrying out heat treatment. In cases where a nitrogen purge was carried out, Grade 3 nitrogen was used; testing was performed after thorough deaeration followed by purging. The cores that had been heat-treated for a given period of time were removed and fully cooled at room temperature (24° C.), following which the various measurements were carried out.

Analysis of Volatile Substances

Sample injection was carried out with the help of a Gerstel thermal desorption system (TDSA/CIS4 System), and a gas chromatograph-mass spectrometer (GC-MS) consisting of an HP5973 mass selective detector system and an HP6890 gas chromatograph, both from Agilent Technology, was used to carry out measurement for 10 minutes at 250° C. in a helium (He) atmosphere and at a sample weight of 1 mg. The measurement sample was cut from the center portion of the core. Identification of the detected ingredients was carried out by comparison with a library, and the compounds inferred to have the highest probabilities were selected. When carrying out quantitative analysis, a working curve was created with a known concentration of toluene and, using the sensitivity ratios with various standard samples, the volatilized amount per unit weight of the sample was calculated. In Tables 2 and 3, “acrylic acid” is a decomposition product of zinc acrylate, “acetophenone” and “α-cumyl alcohol” are decomposition products of dicumyl peroxide (used here under the trade name “Percumyl D”), and “tert-butanol” is a decomposition product of 1,1-di(t-butylperoxy)cyclohexane (used here under the trade name “Perhexa C”).

Thermogravimetric Analysis (TGA)

Using a TG-8120 thermogravimetric/differential thermal analyzer from Rigaku Corporation, the amount of decrease was measured at a temperature rise rate of 10° C./min and a holding time at 210° C. of 18 hours. This measurement was carried out in a nitrogen atmosphere, using Al₂O₃ as the standard sample, and at a sample weight of 10 mg. The measurement samples were cut from the center portion of the core and from the core surface.

Formation of Cover (Intermediate Layer and Outermost Layer)

Next, an ionomer resin material (an ionomer compound of Himilan 1605, Himilan 1706 and Himilan 1557 from DuPont-Mitsui Polychemicals Co., Ltd. was used; the Shore D hardness of the resin material was 63) was injection-molded as the intermediate layer material over the cores obtained as described above, thereby encasing the cores and giving intermediate spherical bodies having an intermediate layer thickness of 1.68 mm. The intermediate spherical body was then set in a different injection mold and a polyurethane resin material (a urethane compound of Pandex T8283, Pandex T8290 and Pandex T8295 from DIC Bayer Polymer, Ltd.; the Shore D hardness of the resin material was 47) was injection-molded as the outermost layer material, thereby encasing the intermediate spherical body and producing three-piece solid golf balls having an outermost layer thickness of 0.8 mm. During this injection-molding operation, specific dimples were formed on the cover surface.

The ball deflection and initial velocity for each of the resulting golf balls were evaluated by the following methods. The results are shown in Tables 2 and 3.

Golf Ball Deflection

The golf ball was placed on a hard plate and the deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The deflection here is the measured value obtained after holding the golf ball isothermally at 23.9° C.

Initial Velocity of Golf Ball

The initial velocity of the golf ball was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The golf ball to be measured was held isothermally at a temperature of 23±1° C. for at least 3 hours, and measurement was carried out in a chamber at a room temperature of 23±2° C. Twenty golf balls were each hit twice. The time taken for the golf ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity.

TABLE 2 Working Example 1 2 3 4 5 6 7 8 9 Core used I I I I I I I I I Heat treatment Treatment 175 175 210 210 250 250 175 210 175 temperature (° C.) Treatment time 6 12 1 6 1 4 6 1 6 (h) Cumulative 486 538 420 583 500 651 486 420 486 temperature Atmosphere air air air air air air air air air Pressure (Pa) 100 100 100 100 100 100 5000 5000 atmospheric Volatile component Acrylic acid 0.210 0.186 0.203 0.158 0.194 0.165 0.224 0.220 0.225 analysis (wt %) Acetophenone 0.040 0.035 0.039 0.032 0.039 0.034 0.041 0.040 0.042 α-Cumyl alcohol 0.083 0.079 0.087 0.063 0.089 0.070 0.089 0.087 0.094 tert-Butanol 0.007 0.005 0.006 0.005 0.005 0.004 0.007 0.008 0.007 Volatile Acrylic acid 0.011 0.010 0.011 0.008 0.010 0.009 0.012 0.012 0.012 component ratio Acetophenone 0.067 0.058 0.065 0.053 0.065 0.057 0.068 0.067 0.070 α-Cumyl alcohol 0.138 0.132 0.145 0.105 0.148 0.117 0.148 0.145 0.157 tert-Butanol 0.028 0.021 0.023 0.020 0.020 0.017 0.030 0.031 0.030 Weight loss (wt %) Core center 1.22 1.06 1.24 1.02 1.23 1.01 1.25 1.24 1.26 Core surface 1.33 1.21 1.33 1.14 1.33 1.17 1.36 1.33 1.35 Core center/ 0.917 0.876 0.932 0.895 0.925 0.863 0.919 0.932 0.933 Core surface Ball deflection (mm) 3.5 3.4 3.5 3.3 3.5 3.3 3.7 3.6 3.6 Ball initial velocity 77.3 77.5 77.3 77.5 77.3 77.6 77.2 77.3 77.2 (m/s) Working Example Comparative Example 10 11 12 1 2 3 4 Core used I I I I I I I Heat treatment Treatment 210 175 210 — 130 170 170 temperature (° C.) Treatment time 1 6 1 — 4 1 1 (h) Cumulative 420 486 420 — 338 340 340 temperature Atmosphere air nitrogen nitrogen — air air air Pressure (Pa) atmospheric atmospheric atmospheric — 100 100 atmospheric Volatile component Acrylic acid 0.218 0.224 0.220 0.242 0.236 0.238 0.240 analysis (wt %) Acetophenone 0.042 0.041 0.042 0.048 0.046 0.047 0.046 α-Cumyl alcohol 0.087 0.101 0.112 0.180 0.170 0.172 0.172 tert-Butanol 0.007 0.008 0.008 0.012 0.011 0.011 0.012 Volatile Acrylic acid 0.012 0.012 0.012 0.013 0.013 0.013 0.013 component ratio Acetophenone 0.070 0.068 0.070 0.080 0.077 0.078 0.077 α-Cumyl alcohol 0.145 0.168 0.187 0.300 0.283 0.287 0.287 tert-Butanol 0.029 0.031 0.032 0.050 0.046 0.047 0.048 Weight loss (wt %) Core center 1.25 1.26 1.23 1.39 1.35 1.34 1.35 Core surface 1.33 1.35 1.33 1.43 1.39 1.39 1.38 Core center/ 0.940 0.933 0.925 0.972 0.971 0.964 0.978 Core surface Ball deflection (mm) 3.6 3.6 3.6 3.7 3.7 3.7 3.7 Ball initial velocity 77.2 77.3 77.2 77.0 77.0 77.0 77.0 (m/s)

TABLE 3 Working Example Comparative Example 13 14 15 16 17 18 19 5 6 7 Core used II II II II II II II II II II Heat treatment Treatment 130 175 210 250 175 175 175 — 130 170 temperature (° C.) Treatment time (h) 24 6 1 1 6 6 6 — 4 1 Cumulative 372 486 420 500 486 486 486 — 338 340 temperature Atmosphere air air air air air air nitrogen — air air Pressure (Pa) 100 100 100 100 5000 atmospheric atmospheric — 100 100 Volatile Acrylic acid 0.216 0.228 0.234 0.225 0.233 0.234 0.230 0.300 0.290 0.287 component Acetophenone — — — — — — — — — — analysis α-Cumyl alcohol — — — — — — — — — — (wt %) tert-Butanol 0.025 0.028 0.023 0.023 0.030 0.029 0.031 0.055 0.051 0.048 Volatile Acrylic acid 0.010 0.010 0.010 0.010 0.010 0.010 0.010 0.013 0.013 0.013 component Acetophenone — — — — — — — — — — ratio α-Cumyl alcohol — — — — — — — — — — tert-Butanol 0.025 0.028 0.023 0.023 0.030 0.029 0.031 0.055 0.051 0.048 Weight loss (wt %) Core center 1.22 1.24 1.21 1.23 1.24 1.25 1.25 1.40 1.39 1.38 Core surface 1.33 1.36 1.33 1.35 1.35 1.35 1.36 1.42 1.42 1.42 Core center/ 0.917 0.912 0.910 0.911 0.919 0.926 0.919 0.986 0.979 0.972 Core surface Ball deflection (mm) 3.3 3.2 3.2 3.2 3.2 3.2 3.2 3.4 3.4 3.4 Ball initial velocity (m/s) 77.3 77.4 77.4 77.5 77.3 77.3 77.2 77.0 77.0 77.0

In Tables 2 and 3, “cumulative temperature” is the value calculated from K×(2+Log T), where K is the heating temperature (° C.) and T is the heating time (h).

As is apparent from Tables 2 and 3, the golf balls in the Working Examples of the invention, both when the core used was I and when it was II, had lower energy losses and were able to achieve higher initial velocities than the golf balls in the Comparative Examples.

Japanese Patent Application No. 2016-124559 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A golf ball comprising a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition comprising (A) a base rubber, (B) an unsaturated carboxylic acid or a metal salt thereof or both, and (C) an organic peroxide; and, letting the content of component (B) in the rubber composition per 100 parts by weight of component (A) be X parts by weight and the concentration of volatile substances from component (B) within the core be A wt %, the value A/X is 0.0123 or less.
 2. The golf ball of claim 1 wherein dicumyl peroxide is used as component (C) in the rubber composition and, letting the content of component (C) in the rubber composition per 100 parts by weight of component (A) be Y parts by weight and the concentrations of acetophenone and α-cumyl alcohol, which are volatile substances from component (C), within the core be respectively B1 wt % and B2 wt %, the value B1/Y is 0.072 or less and the value B2/Y is 0.270 or less.
 3. The golf ball of claim 1, wherein the concentrations of acetophenone and α-cumyl alcohol, which are volatile substances from component (C), are respectively 0.043 wt % or less and 0.15 wt % or less.
 4. The golf ball of claim 1, wherein 1,1-di(t-butylperoxy)cyclohexane is used as component (C) in the rubber composition and, letting the content of component (C) in the rubber composition per 100 parts by weight of component (A) be Y parts by weight and the concentration of tert-butanol, which is a volatile substance from component (C), within the core be D wt %, the value D/Y is 0.033 or less.
 5. The golf ball of claim 1, wherein the concentration of tert-butanol, which is a volatile substance from component (D), is 0.033 wt % or less.
 6. A golf ball comprising a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition comprising (A) a base rubber, (B) an unsaturated carboxylic acid or a metal salt thereof or both, and (C) an organic peroxide; and the concentration of acrylic acid, which is a volatile substance from component (B), within the core is 0.234 wt % or less.
 7. A golf ball comprising a core and a cover of at least one layer encasing the core, wherein the core is a product molded under heat from a rubber composition comprising (A) a base rubber, (B) an unsaturated carboxylic acid or a metal salt thereof or both, and (C) an organic peroxide; and, in thermogravimetric analysis of the core, letting (a) represent the percent weight loss at a center of the core and (b) represent the percent weight loss at a surface of the core, the value (a) is 1.31 wt % or less, the value (b) is 1.37 wt % or less, and the value (a)/(b) is 0.96 or less. 